Laser -induced breakdown detection was perfomed to monitor the nanoparticles in an acqueous solution by means of a two-dimensional optical imaging method. To verify the relationship between the particle size and the optical image of a laser-induced plasma, we investigated the characteristics of its spatial distribution corresponding to the number of breakdown events plotted on the laser beam propagating to the number of breakdown events plottted on the laser beam propagating axis. It was found that, for particles smaller than 50 nm in diameter, the spatial distribution follows a single Gaussiann curve. For particles in the diameter range from 100 to 1000 nm, however the spatial distribution follows a sum of the multiple Gaussian curves with different peak positions and peak heights. We demonstrated that particles smaller than 20nm in trace concentrations, which are mixed with larger particles in the diameter range of a new hundred nm, can be misuread by a peak deconvolution of the spatial distribution of a laser-induced plasma.